1. Technical Field
The present invention, in one respect, relates to the electrolytic treatment of an acid solution, for instance the recovery of metals from an acid solution. One example of this aspect of the present invention is the preparation of a more concentrated solution containing hexavalent chromium from a dilute electroplating rinse solution containing hexavalent chromium. The present invention, in another respect, relates to the electrolytic treatment of an acid bath such as an electroplating bath or anodizing bath for the purpose of rejuvenating the bath. This treatment is in combination with treatment of an acid solution, wherein the acid solution is a rinse solution for the acid bath.
2. Description of the Prior Art
In the electroplating of a workpiece in a chromic acid solution, the electroplating cell is generally followed by one or more rinse tanks in which the plated workpiece is rinsed. It is desirable to maintain a low concentration of chromium ions in the rinse water. Accordingly, where more than one rinse tank is used, fresh water can be introduced into the last rinse tank, and cascaded from the last rinse tank to the penultimate rinse tank, on up to the rinse tank closest to the electroplating cell. The rinse tank closest to the electroplating cell experiences a build-up of chromium ions in the tank. The rinse solution in this rinse tank has too high a concentration of chromium ions for sewer disposal of the solution. In addition, it is economically desirable to recover the chromium ions if possible.
U.S. Pat. No. 4,302,304 discloses a process for treating a chromic acid-containing metal plating waste water. The metal plating waste water is fed to the cathode chamber of an electrolytic cell. The cell is partitioned with a diaphragm. A DC voltage is applied between the cell anode and the cathode which impresses a current across these electrodes. This causes the migration of chromate or dichromate ions to the anode chamber. Chromic acid is recovered in the anode chamber of the cell, and reusable water is recovered in the cathode chamber of the cell. The diaphragm may be made of glass fiber, porcelain, cloth, or of porous high molecular weight polymers. The chromic acid withdrawn from the anode chamber is sufficiently concentrated that it can also be reused.
U.S. Pat. No. 3,481,851 discloses reconditioning a chromic acid containing metal solution such as a used chrome plating solution. The used solution is introduced as anolyte into an anode compartment of an electrodialysis cell. The cell has a cation permeable membrane dividing the anode compartment from a cathode compartment in the cell. When the cell is energized, dissolved foreign ions in the used solution, such as copper, iron, zinc, nickel and cadmium, selectively pass through the membrane to the cathode compartment, and simultaneously, oxygen evolved at the anode oxidizes trivalent chromium to hexavalent chromium. The catholyte is an acid solution such as one containing 10% by volume of hydrochloric acid.
Similar disclosures are contained in U.S. Pat. Nos. 3,764,503, 4,006,067, 4,243,501, 4,337,129, and 4,857,162.
U.S. Pat. No. 3,948,738 discloses, in one embodiment, introducing a diluted exhausted chromium plating solution into an anode compartment of a two-compartment cell. A more concentrated exhausted chromium plating solution is introduced into the cathode compartment. On energizing the cell, chromic acid values transfer to the anolyte. The cell is deenergized, and the anolyte is withdrawn for use in the chromium plating bath. The catholyte is transferred to the anode compartment and electrolysis is resumed. The purpose of dilution of the anolyte is to maintain a low concentration of iron in the chromium plating bath.